In recent years, power grids have increasingly used wind turbine generators, photovoltaic power generation, or other generators based on renewable energy. It is demanded that such power grids make effective use of electrical power generated by renewable energy.
However, when a generator based on renewable energy is used, the amount of electrical power generated by such a generator varies with solar radiation, wind conditions, and other weather conditions that are difficult to predict accurately. This causes power supply and demand imbalance. This imbalance causes fluctuation in system frequency, thereby incurring power quality deterioration.
In general, an energy storage battery is added to each unit or a plurality of units of a renewable energy in order to inhibit generated electrical power from varying with the weather conditions. The energy storage battery compensates for deficiency of generated electrical power or sharp output changes in the renewable energy that are caused by changes in the weather conditions. In this instance, it is ideal that an energy storage battery capable of compensating for the whole output change in the renewable energy be added. However, such an energy storage battery costs too much, thereby increasing the unit cost of renewable energy system power generation.
As described above, at a site where generators based on renewable energy are installed, every effort is made, wherever possible, to suppress or absorb changes in generated electrical power that are caused by the changes in the weather conditions. However, changes in the generated electrical power that cannot be suppressed or absorbed are to be compensated for by existing thermal power generators and hydroelectric power generators included in an employed power grid. More specifically, the existing thermal power generators and hydroelectric power generators receive an automatic frequency control signal (AFC signal) from an automatic dispatching system and functions to absorb changes in the electrical power generated by the generators based on renewable energy.
However, when the proportion of the renewable energy based generators increases within the power grid, the capacity furnished by the existing thermal power generators and hydroelectric power generators becomes insufficient.
“Normal/Emergency Load Frequency Control in Power grid” by Power grid Technical Committee of Electrical Power Energy Division of the Institute of Electrical Engineers of Japan (Technical Report of the Institute of Electrical Engineers of Japan, No. 869) describes a method for determining an available wind turbine generator connection capability interconnectable to the power grid from the relationship between a power capacity providable by existing thermal power generators and hydroelectric power generators and the power generation capacity of renewable energy based generators that varies with weather conditions.
Japanese Patent No. 4559577 discloses a control method for selecting a wind turbine generator exhibiting a significant output fluctuation from a plurality of wind turbine generators and suppressing the output of the selected wind turbine generator to smooth the power outputs of the wind turbine generators. If the utilization factors of the wind turbine generators that prevail during a predetermined period are calculated and the utilization factor of a particular wind turbine generator is found to be lower than a threshold value, the control method is exercised to make the utilization factors of all the wind turbine generators nearly equal by excluding the particular wind turbine generator from wind turbine generators to be subjected to output suppression.
In a power grid that is operated while a plurality of wind turbine generators, photovoltaic power generation, and other renewable energy based generators are connected, it is desired that the power grid be steadily operated while making the most of the renewable energy based generators.
However, even when the method described in Japanese Patent No. 4559577 is exercised after the method described in “Normal/Emergency Load Frequency Control in Power grid” by Power grid Technical Committee of Electrical Power Energy Division of the Institute of Electrical Engineers of Japan (Technical Report of the Institute of Electrical Engineers of Japan, No. 869) is used to determine the available wind turbine generator connection capability interconnectable to the power grid, the power grid cannot be steadily operated while making the most of the renewable energy based generators.
There are various problems with the determination of the available wind turbine generator connection capability. When, for instance, the available wind turbine generator connection capability is calculated in a power grid state where the demand is lowest (e.g., in winter), basic information (allowable control deviation, available control capability, and load fluctuation) for determining the available wind turbine generator connection capability is different from the basic information for another power grid state where the demand is high (e.g., in summer). Further, although the available wind turbine generator connection capability is constantly changing due to the occurrence of system faults and changes in the load fluctuation of users connected to the power grid, the wind turbine generator connection capability becomes excessive or inadequate.
Furthermore, the electrical power generated by a wind turbine generator depends on weather conditions so that a time zone where the wind turbine generator can be operated at full rated capacity is extremely limited. Therefore, even when the capacity of a system-interconnected wind turbine generator is made lower than the available wind turbine generator connection capability, the wind turbine generator is always operated at a capacity lower than the available wind turbine generator connection capability due to the weather conditions. This makes it impossible to make effective use of the renewable energy. For example, even if the total rated power generation capacity of a plurality of system-interconnected wind turbine generators is suppressed to 90%, which is not higher than the available wind turbine generator connection capability, the rated power generation output of the wind turbine generators cannot always be obtained due to the weather conditions. Hence, the wind turbine generators are operated at 50% of their capacity, which is lower than the available wind turbine generator connection capability. This makes it impossible to make the most of the renewable energy.
Moreover, the renewable energy can be effectively used when the overall output fluctuation of all renewable energy is suppressed instead of operating the renewable energy in such a manner as to suppress the output fluctuation of each renewable energy. However, such an overall control scheme is not described in Japanese Patent No. 4559577.
The present invention has been made in view of the above circumstances and provides a power grid operation control system, device, and method for steadily operating a power grid while making the most of renewable energy.